This disclosure generally relates to the use of fasteners to secure two or more structures or workpieces (at least one of which is made of composite material, such as fiber-reinforced plastic) in a manner such that high interference fit of the fasteners within their respective holes in the structures is achieved. In particular, this disclosure relates to apparatus and methods for fastening a layer of composite material to another layer of material using an interference fit fastener assembly having a bolt or a pin and a mating part (e.g., a nut or a collar).
As used herein, the category “mating parts” comprises internally threaded nuts and collars and swaged collars. As used herein, the category “fasteners” includes bolts and pins. As used herein, the term “external projections” should be construed broadly to encompass at least the following types: (1) external threads and (2) external annular rings. As used herein, the term “hole” means a surface that bounds a space having openings at opposing ends. In the context of fiber-reinforced plastic material, the surface bounding the space may be formed by resin and fibers.
At least one method for fastening multiple layers of material together is to clamp up the layers, drill holes, and then insert some type of fastener into the holes and thereby secure the layers together. The fasteners are usually inserted in a net or clearance fit in the receiving holes in the layers. For many applications, this will be sufficient. However, when the assembled structure is subjected to cyclic loading, the looseness of the fit of the fasteners within their holes can result in continual working of the fasteners within their holes.
Additional challenges are presented when one or more of the fastened layers are made of composite material. For example, carbon fiber-reinforced plastics (CFRP) are considerably less conductive than metal. Electrical current is conducted through carbon fibers in the CFRP structure. Any discontinuity between the carbon fibers and the metallic sleeve or pin in the CFRP hole is undesirable. One way to avoid discontinuity in CFRP joints is to incorporate sleeved fasteners installed in an interference fit condition to achieve closer proximity of the carbon fiber to the fastener.
An interference fit of the fastener (hereinafter “interference fit fastener”) in the hole can effectively reduce discontinuities due to cyclic loading of the assembled structure. Interference creates a tighter joint that reduces movement, resulting in enhanced fatigue performance. Additionally, interference fit fasteners can help ensure safe dissipation of electrical current.